Secondary Electron Transmission Studies of the Electron Diffusion and Thermalization Processes in Thin CVD Diamond Films
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Secondary Electron Transmission Studies of the Electron Diffusion and Thermalization Processes in Thin CVD Diamond Films Joan E. Yater, Kevin L. Jensen, Tatyana I. Feygelson, and Bradford B. Pate Naval Research Laboratory, Washington, DC 20375, U.S.A. ABSTRACT Secondary electron transmission measurements are used to examine the electron diffusion and thermalization processes in B-doped nanocrystalline diamond membranes of thickness 2.3 μm and 650 nm. Specifically, transmitted energy spectra are measured following impact by an electron beam that penetrates deeper in the membrane as the beam energy Eo increases. A fullythermalized emission peak is observed for Eo ≤ 16 keV and Eo ≤ 7 keV from the 2.3-μm-thick and 650-nm-thick films, respectively, with a small high-energy tail beginning to emerge at higher Eo. These measurements are analyzed using Monte Carlo simulations that generate constant-energy contour lines (down to 0.05 Eo) as a function of beam depth into the film. From these simulations, we deduce that secondary electrons have a minimum thermalization length of ~ 630 nm and ~ 260 nm in the 2.3-μm-thick and 650-nm-thick films, respectively. Further insight into the secondary-electron transport behavior is gained from the analysis, and this understanding is applied to the design of a transmission electron-current amplifier device. INTRODUCTION Diamond has a variety of electronic properties that make it an appealing material for advanced solid-state electronic devices and for integration with vacuum electronics. In particular, the negative electron affinity (NEA) at hydrogenated surfaces makes diamond a uniquely promising vacuum electron emitter material. While efforts have been underway to exploit this property to achieve high emission current, there are applications for which narrow energy spread is also critical to device performance. As such, the electron transport processes that influence the emitted energy distribution must be well understood. The objective of this study is to gain such an understanding that can then be used to guide in the development of a novel diamond-based electron emitter device. This device, a transmissionmode electron-current amplifier, will be used to multiply the beam current produced by a primary cathode while also reducing the energy spread. This process is shown schematically in figure 1a. Briefly, a primary beam impacts the diamond and penetrates a finite distance into the film. During penetration, the electrons lose energy by exciting valence electrons into the conduction band, with the internal current gain being proportional to the incident beam energy Eo [1, 2]. The secondary electrons then diffuse through the film while losing energy via phonon scattering, and the thermalized electrons that reach the NEA surface are emitted into vacuum. In our previous secondary-electron-emission (SEE) studies, very high emission gains (~ 40 130) were measured from NEA diamond [3]. However, these studies used a reflection configuration for which the beam penetration depth / electron escape
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